1. Field of the Invention
The present invention relates to a switching regulator, control method thereof and power-supply device including the switching regulator. In particular, the present invention relates to a technique which controls an oscillating frequency of a nonlinear control method switching regulator.
2. Description of the Related Art
A nonlinear control method such as hysteresis control having a high response speed to a rapid load change has been recently adopted in the field of portable electronic devices including an industrial electronic device. The nonlinear control method controls ON/OFF of a switching element by directly comparing an output voltage with a reference voltage in a comparator without using an air lamp. With this configuration, a very high response speed is obtained in the nonlinear control because of the absence of delay due to the frequency characteristic of the air lamp generating in linear control and the absence of delay of a wasted time for one cycle of a switching operation. For example, JP4107209 B discloses a nonlinear control method switching regulator.
Stable switching with a high switching frequency may be requested for a nonlinear control method switching regulator. However, an equivalent series resistance (ESR) is very small, and an output voltage ripple is significantly reduced when a ceramic capacitor is used. For this reason, stable switching with a high switching frequency can not be performed. Consequently, a technique is used, which inputs to a comparator a composite voltage of an output voltage and a ripple voltage similar to an inductor current, and compares the composite voltage with a reference voltage in the comparator so as to control ON/OFF of a switching element. With this technique, stable switching can be achieved even with a high switching frequency. For example, JP3981083B discloses one example of the above technique.
In general, the switching frequency of a switching regulator fluctuates based on constants of an inductor and an output capacitor, input and output conditions of input and output voltages, or the like in a nonlinear control method without using an air lamp.
For this reason, characteristics of several applications including a communication device are deteriorated by the generation of undesired electromagnetic interference due to the fluctuation in the switching frequency.
FIG. 8 provides a circuit view of a nonlinear control method switching regulator according to a conventional example. FIG. 9 provides a timing chart of each signal of the switching regulator in FIG. 8. The operation of the switching regulator in FIG. 8 will be hereinbelow described.
In FIG. 8, the switching regulator according to the conventional example includes a comparator 1, reference voltage generation circuit 2, driving circuit 3, ripple generation circuit 4, voltage composite circuit 5, switching elements M1, M2, inductor L1 and capacitor C1. A load 100 is connected to an output terminal of the switching regulator. In this case, a feedback voltage Vfb is a composite voltage of an output voltage Vout and a ripple voltage Vripple similar to an inductor current. The ripple voltage Vripple is generated in the ripple generation circuit 4. The DC output voltage Vout sets the composite voltage of the output voltage Vout and the AC component of the ripple voltage Vripple as Vfb to be set only by the reference voltage Vref.
In FIG. 9, the ripple of the ripple voltage Vripple is illustrated as being considerably larger than the ripple of the output voltage Vout, and the ripple of the feedback voltage Vfb is illustrated as being the same as the ripple of the ripple voltage Vripple.
In FIG. 8, the comparator 1 compares the feedback voltage Vfb with the reference voltage Vref. When the feedback voltage Vfb drops below the reference voltage Vref, the output signal CMPOUT of the comparator 1 becomes a low level from a high level after a response delay time temp, In this case, the reference voltage Vref is generated in the reference voltage generation circuit 2. When the low level output signal CMPOUT is input to the driving circuit 3, a control signal PHSIDE controlling a switching element M1 becomes a low level, and a control signal NLSIDE controlling a switching element M2 becomes a low level. Namely, the switching element M1 is turned on, and the switching element M2 is turned off. A current is supplied to an inductor L1 from an input voltage Vin in response to the turning-on of the switching element M1, and an inductor current IL increases with the inclination of (Vin−Vout)/L.
When the inductor current IL exceeds an output current lout upon the increase in the inductor current IL, a current flows in the capacitor C1, an electric charge is accumulated in the capacitor C1, and the output voltage Vout increases. The comparator 1 compares the reference voltage Vref with the feedback voltage Vfb. When the feedback voltage Vfb exceeds the reference voltage Vref, the output signal CMPOUT of the comparator 1 becomes a high level from a low level after the response delay time temp. When the high level output signal CMPOUT is input to the driving circuit 3, the control signal PHSIDE controlling the switching element M1 becomes a high level, and the control signal NLSIDE controlling the switching element M2 becomes a high level. Namely, the switching element M1 is turned off, and the switching element M2 is turned on. A current flows in the inductor L1 from a ground in response to the turning-on of the switching element M2, and the inductor current IL decreases with the inclination of Vout/L.
When the inductor current IL drops below the output current lout upon the decrease in the inductor current IL, a current flows in the load 100 from the capacitor C1, and the output voltage Vout decreases. With the repetition of such an operation, the switching regulator in FIG. 8 stably outputs the output voltage.
As described above, according to the switching regulator of the conventional example, the oscillating frequency of the switching regulator is mostly determined based on the response delay time temp of the comparator 1. Namely, when the response delay time temp of the comparator 1 fluctuates, the oscillating frequency of the switching regulator fluctuates. The response delay time temp is affected by the amplitude and slew rate of the feedback voltage Vfb input to the comparator 1 in addition to the response characteristic of the comparator 1. The ripple voltage Vripple and the output voltage Vout fluctuate based on the input and output conditions of the input and output voltages and the constants of the capacitor and the inductor. Therefore, the oscillating frequency of the switching regulator fluctuates based on the input and output conditions of the input and output voltages and the constants of the capacitor and the inductor.